# Two Round Feistel network

While reading on block ciphers and DES I read that two-round Feistel network is not a secure PRP? Is there any easy to understand proof to explain the intuition behind this statement. I did search around and also reviewed this question but wasn't really able to understand why it isn't secure PRP. Any help would be much appreciated!

• The key-word is Luby and Rackoff construction – kelalaka Oct 25 '20 at 9:07
• Yes I read that and it didn't make sense to me maybe because of the notations used here: crypto.stanford.edu/pbc/notes/crypto/prp.html Is there an easy to understand explanation like an example or proof? – Alex Oct 25 '20 at 9:10
• – kelalaka Oct 25 '20 at 11:02

It holds $$P_L\oplus C_L=F_0(P_R)$$. That implies a distinguishable property: for any fixed $$P_R$$ and whatever the round function $$F_0$$, when we flip bit(s) in $$P_L$$, that flips the corresponding bit(s) in $$C_L$$ and leaves the other bit(s) in $$C_L$$ unchanged.